In steel mill, automotive and machining industries, significant quantities of lubricants, cutting oils and cutting fluids are employed. These oil-containing waters, when spent, are sent to the plant's waste treatment facility where they are typically combined with other plant waste streams. In simple form, these "oily" waters contain undiluted petroleum oils and an emulsifier. However, in practice they typically contain fatty acids, surfactants, biocides, antioxidants and many other additives. Synthetic oils may also be present. Additionally, oily waste waters from these plants typically have a wide variety of other contaminants, including solids, metal fines, and sometimes dissolved metals. Iron and copper are the metals seen most often because equipment metallurgies are often fabricated from these metals.
In view of recent rigid environmental standards, increases in oil prices and decreases in oil supply, it has become very important to recover these oils for reuse. In the case of oil-in-water emulsions, de-emulsification may be achieved by acid addition, use of primary inorganic coagulants such as ferric sulfate or alum, or organic polymeric emulsion breakers. Often, oil skimmings resulting from oil-in-water destabilizations are emulsions too, but converted into a water-in-oil emulsion with a much higher oil content. Chemicals break emulsions by neutralizing repulsive charges between particles, precipitating or salting out the emulsifying agent and/or altering the interfacial film so it is readily broken. Reactive cations (H+, Al+3, Fe+3, cationic polymers) are particularly effective in breaking dilute oil in water emulsions. Once the charges have been neutralized and the interfacial film broken, the small oil droplets are able to coalesce upon collision with other droplets. The inorganic coagulants also provide a hydroxide floc for the adsorption of oil. While this aids in oil removal, it also increases the sludge volume, thus complicating oil recovery and creating a disposal problem. Finally, oil is extremely hard to extract from aluminum hydroxide floc.
Progressively stricter regulatory criteria have forced industry to drastically reduce the residual metal content in wastewater discharges, while the increased cost of disposal of solid metal wastes has forced industries to reduce their sludge volume. Almost all metals of concern precipitate as a hydroxide. However, they all reach minimum solubility at different pH values. A compromise pH must be chosen if removing several metals. The efficiency of metals removal will be limited by reaction time, hydroxide concentrations and settling rates. In addition, hydroxide precipitation and most other conventional methods of heavy metals removal produce copious amounts of sludge, usually classified as hazardous material. Heavy metal precipitants such as dithiocarbamates have the advantage that they produce a more compact sludge, but they are expensive and highly toxic.